1. Field of the Invention
The present invention relates to an electro-optical device represented by a liquid crystal display device, and to the structure of an electronic apparatus provided with as a part such an electro-optical device. Incidentally, the term "semiconductor device" in this specification designates devices in general which can function with the use of characteristics of semiconductor, and therefore the above-described electro-optical device and electronic apparatus fall into the semiconductor devices.
2. Description of the Related Art
In recent years, what has received attention is an active matrix type liquid crystal display device in which a circuit is consisted of a thin film transistor (hereinafter, referred to as TFT) utilizing a polysilicon film. This is a device intended for realization of image display with high definition by, using a plurality of pixel TFTs that are arranged in matrix, controlling on matrix basis the electric field that applies on the liquid crystal.
In such an active matrix type liquid crystal display device, a capacitance (condenser, or capacitor) is comprised of a pixel electrode formed for every pixel and an opposite electrode formed on the side opposite thereto through the liquid crystal placed in between. However, the capacitance having those alone is not large enough and hence is ordinarily supplemented with a storage capacitance (also called as Cs) formed separately from the former.
There are various structures for the storage capacitance (Cs structure), among which presented (in Japanese Patent Application Laid-open Nos. Hei 8-43854 and 8-306926) is the structure where an insulating film is, taking into consideration the aperture ratio in a transmission type liquid crystal display device, sandwiched between two layers of transparent conductive films.
Each of the above publications claims that with its Cs structure, a large capacitance can be ensured without degrading the aperture ratio through employment of a transparent conductive film, such as ITO, for both of two electrodes that form a pair and constitute the storage capacitance. The conventional structure is shown in FIG. 2.
In FIG. 2, reference numeral 201 denotes a capacitance electrode formed of the transparent conductive film. An interlayer insulating film 202 is formed thereon which serves also as a dielectric material of the storage capacitance, and a pixel electrode 203 is further formed.
However, such a structure may have a problem of poor coverage of the interlayer insulating film 202 at an end portion 204 (the area circled with a dotted line) of the storage capacitance. That is, though thinness is preferable as the dielectric material to enlarge the capacitance, it causes a problem of the short circuit between the capacitance electrode 201 and the pixel electrode 203. For that reason, there is a relationship of trade-off between the thickness of the dielectric material and enlargement of the capacitance, where a certain degree of film thickness is required in order to carry out the function as the interlayer insulating film.
For instance, the film thickness of the capacitance electrode 201 is required to be about 100 to 200 nm because of the transparent conductive film having a resistance higher than that of a metal film and in view of the electric potential distribution. To completely cover the capacitance electrode 201, it therefore is necessary for the interlayer insulating film to have a film thickness of at least 200 nm or more. However, the volume of the capacitance is in inverse proportion to the film thickness of the dielectric material, and to thicken the film thickness thereof is not desirable when aiming at securement of a large capacitance.
As described above, many problems are still remained unresolved under the existing circumstances though sandwiching the interlayer insulating film between a pair of transparent conductive films makes it possible to enlarge, without degrading the aperture ratio, an area where the storage capacitance may be formed.